1. Field of the Invention
The present invention relates to an optical head used upon recording and/or reproducing an information signal in and/or from an information recording medium such as an optical disc, and an optical disc device equipped with such an optical head.
2. Description of the Background Art
So-called three-beam optical heads have been conventionally known. In such a conventional optical head, a laser light emitted from a laser light source is split by a diffraction element into a 0th-order diffracted light (hereinafter, “main beam”) that is not diffracted and two ±1st-order diffracted lights (hereinafter, “two sub-beams”) that are diffracted and polarized, and the main beam and two sub-beams are focused on an optical disc by an objective lens, thereby forming three spots including one main spot and two sub-spots. The two sub-spots are arranged at positions distanced from the main spot in inner and outer radial directions of the optical disc by half the groove pitch. The main beam and two sub-beams reflected by the optical disc are respectively introduced to three four-divided photodetectors (two-divided in radial direction and two-divided in track direction) disposed at different positions on a light receiving element.
A tracking error signal (hereinafter, “TE signal”) by a differential push-pull (DPP) method can be obtained by subtracting a signal obtained by a sum signal of a push-pull signal detected by the four-divided photodetector having received one sub-beam (hereinafter, “signal PPS1”) and a push-pull signal detected by the four-divided photodetector having received the other sub-beam (hereinafter, “signal PPS2”) by a specified gain from a push-pull signal detected by the four-divided photodetector having received main beam (hereinafter, “signal PPM”).
Offset components are generated in the above signal PPS1, signal PPS2 and signal PPM resulting from the displacement of the objective lens to follow the movement of an information track caused by the center-deviation of the optical disc. The offset components contained in the signal PPS1, signal PPS2 and signal PPM have the same polarity, the signal PPS1 and signal PPS2 are in opposite phases and the signal PPS2 and signal PPM are in opposite phases due to the arrangement (½ groove pitch) of the sub-beams in radial direction. Thus, the TE signal having the offset components canceled out can be obtained by subtracting the sum signal of the signal PPS1 and signal PPS2 from the signal PPM. In accordance with this TE signal, stable tracking servo is performed for the information track of the optical disc.
However, according to the above DPP method, the intervals between the main spot and two sub-beams need to conform to the ½ groove pitch in the radial direction of the optical disc. Conversely speaking, no good TE signals can be obtained for such an optical disc whose groove pitch largely deviates from twice the spot interval. Thus, upon recording/reproducing information in/from a plurality of kinds of optical discs having different groove pitches by means of one optical head using the TE signal by the DPP method, extra means such as the one for rotating the diffraction element for splitting the light beam into the main beam and two sub-beams about an optical axis to change directions of diffracted lights need to be provided in order to conform the spot interval in radial direction to ½ groove pitch based on the groove pitch of the optical disc by discriminating the kind of the optical disc, for example, among a DVD-RAM (storage capacity of 2.6 GB) with a groove pitch of 1.48 μm, a DVD-RAM (storage capacity of 4.7 GB) with a groove pitch of 1.23 μm, a DVD-R (Recordable) and a DVD-RW (Rewritable) with a groove pitch of 0.74 μm as optical discs of DVD (Digital Versatile Disc) standard.
In order to apply the DPP method to optical discs having different groove pitches without providing such means for rotating the diffraction element about the optical axis, there is proposed a technique for arranging a main spot and two sub-beams on the same track using a diffraction grating divided into a plurality of areas in which the phases of periodic structures of grating grooves are respectively different. Such a conventional technique is described below.
FIG. 13A is a plan view showing the periodic structures of grating grooves of a diffraction element (prior art 1) in a conventional optical head, and FIG. 13B is a diagram showing a phase distribution of a light beam diffracted by the periodic structure of the diffraction element of FIG. 13A (see, for example, FIG. 5 of Japanese Unexamined Patent Publication No. H09-81942).
In FIG. 13A, a diffraction element 60 of the prior art 1 includes two areas 61, 62 divided in a direction corresponding to a radial direction of an optical disc. The periodic structure of the grating grooves (shown by hatching) formed in the left area 62 differs from that of the grating grooves (shown by hatching) formed in the right area 61 by a ½ period.
±1st-order diffracted lights diffracted by such periodic structures have a phase distribution as shown in FIG. 13B.
If the phase of the light diffracted by the right area 61 of FIG. 13A is assumed to be a reference (zero), the phase of one of the ±1st-order diffracted lights diffracted by the left area 62 advances by π and the phase of the other retards by π. In other words, if the phase of a right area 65 of FIG. 13B is assumed to be zero, the phase of a left area 66 is π, whereby the phase distribution is comprised of two phases. Further, there is no phase distribution in the main beam which is a 0th-order diffracted light.
Thus, also in the case of arranging the main spot and two sub-beams on the same track, a push-pull signal (signal PPM) detected by the main beam and push-pull signals (signal PPS1, signal PPS2) detected by the two sub-beams are in opposite phases. Accordingly, a TE signal having offset components canceled out can be obtained independently of differences in the groove pitches of the optical discs by subtracting a sum signal of the signal PPS1 and signal PPS2 from the signal PPM.
FIG. 14A is a plan view showing the periodic structures of grating grooves of a diffraction element (prior art 2) in another conventional optical head, and FIG. 14B is a diagram showing a phase distribution of a light beam diffracted by the periodic structure of the diffraction element of FIG. 14A (see, for example, FIG. 7 of Japanese Unexamined Patent Publication No. 2004-145915).
In FIG. 14A, a diffraction element 70 of the prior art 2 includes three areas 71, 72 and 73 divided in a direction corresponding to a radial direction of an optical disc. The periodic structure of the grating grooves (shown by hatching) formed in the right area 72 is shifted downward by a ¼ period from that of the grating grooves (shown by hatching) formed in the middle area 71, and the periodic structure of the grating grooves (shown by hatching) formed in the left area 73 is shifted upward by a ¼ period from that of the grating grooves (shown by hatching) formed in the middle area 71.
Since the phase of the light diffracted by such periodic structures change according to differences in the periodic structures substantially in the same manner as described above, there is a phase distribution as shown in FIG. 14B.
FIG. 14B is a diagram showing a phase distribution of one of ±1st-order diffracted lights, and signs are reversed in a phase distribution of the other light. In FIG. 14B, if the phase of a middle area 75 is assumed to be a reference (zero), the phase in a right area 76 is +π/2 radian (+90°) and that in a left area 77 is −π/2 radian (−90°), whereby there is a phase distribution comprised of three phases. A phase difference between the phases in the left and right areas 76, 77 is π radian, which is the same phase difference as in the prior art 1 described above.
Thus, also in the case of arranging the main spot and two sub-beams on the same track, a push-pull signal (signal PPM) detected by the main beam and push-pull signals (signal PPS1, signal PPS2) detected by the two sub-beams are substantially in opposite phases. Accordingly, a TE signal having offset components canceled out can be obtained independently of differences in the groove pitches of the optical discs by subtracting a sum signal of the signal PPS1 and signal PPS2 from the signal PPM.
However, the above conventional three-beam optical heads have had the following problems.
First of all, the optical head using the diffraction element according to the prior art 1 has a problem that the amplitude of the TE signal largely decreases if the objective lens is displaced in the radial direction of the optical disc.
FIG. 15 is a graph (C1) showing a simulation result of a change in the amplitude of the TE signal by the DPP method in relation to an objective lens displacement in a radial direction in the case of reproducing information from a DVD-RAM with a groove pitch of 1.23 μm as an optical disc using the optical head including the diffraction element according to the prior art 1. It should be noted that vertical axis of FIG. 15 represents normalized amplitude with the amplitude of the TE signal when the objective lens displacement is zero (track center) set as 100%. As can be understood from a curve C1 of FIG. 15, the amplitude of the TE signal decreases about by 30% if the objective lens is displaced from the track center by ±0.2 mm.
Next, in the optical head using the diffraction element according to the prior art 2, a decreasing rate of the amplitude of the TE signal in relation to the objective lens displacement is suppressed low although the amplitude of the TE signal is more decreased as a whole as compared to the prior art 1 as shown by a curve C2 of FIG. 15.
However, the prior art 2 has had a problem that optimal positions of a sub-spots of the two sub-beams on the track differ in optical discs having different groove pitches. The reason for this is described in detail next.
FIG. 16 shows simulation waveforms of push-pull signals obtained from a main beam and two sub-beams in the case of optimally adjusting the arrangement of sub-spots for a DVD-R/RW with a groove pitch of 0.74 μm. Calculation was carried out on the condition that wavelength was 660 nm, NA 0.65 and groove pitch 0.74 μm, and the width of the middle area shown in FIG. 14B was 25% of the diameter of the transmitting beam 74 assuming that the respective beams had the same light amount. It should be noted that the optimal arrangement of the sub-spots is equivalent to an adjustment to maximize the amplitude of the TE signal by rotating the diffraction element for generating the sub-beams about the optical axis. In FIG. 16, the push-pull signals (signal PPS1, signal PPS2) obtained from the two sub-beams overlap because being in the same phase, and the phase thereof is displaced from that of the push-pull signal (signal PPM) obtained from the main beam by π radian.
On the other hand, FIG. 17 shows simulation waveforms of push-pull signals obtained from a main beam and two sub-beams in the case of having the same sub-spot arrangement as in the case of the aforementioned DVD-R/RW for a DVD-RAM with a groove pitch of 1.23 μm. Calculation conditions were the same as in the above case except the groove pitch. In FIG. 17, the signals PPS1, PPS2 have waveforms whose phases are displaced by the same amount in opposite directions from the groove center. Since the signals PPS1, PPS2 are added upon generating the TE signal, such a phase difference is canceled out and there can be obtained a TE signal that zero-crosses at the groove center.
However, if a light amount balance of the two sub-spots is lost due to part errors and assembling errors of the optical head such as the inclination of the optical axis of the emitted laser light and the inclination of the objective lens to differentiate the amplitudes of the signals PPS1, PPS2, such a phase difference cannot be canceled out and the TE signal has such a waveform as to zero-cross at a position displaced from the groove center. With such a TE signal, there has been a problem of reducing the accuracy of a tracking servo.